Muscles are some of the most amazing, dynamic, and adaptable structures in the human body.
I mean, think about this.
We beat the crap out of muscle tissue at the gym or when we workout and then, a day or
two later, those muscles are ready for more.
So, in today's video, we're going to talk about how muscles respond to the many beatings
that they take as well as these amazing adaptations that come from exercise and yes, one of those
adaptations that we'll discuss is how muscles get bigger and stronger but is that the only
way that muscles adapt?
Not even close and we're going to talk about how and why.
So, let's do this.
[Intro]
So, in order for us to understand how muscles adapt, we need to know a little bit about
how they're put together and how they function.
Now, keep in mind, we're going to be focusing on skeletal muscle tissue throughout this
video which the name implies the majority of these skeletal muscles attached to and
move the skeleton but there's actually two other types of muscle tissue throughout the
body and that's our video quiz question of the day.
Can you name those other two types of muscle tissue?
If you can, post them in the comments below and we'll pin the correct answer at the top
of the comment section but back to skeletal muscles.
Let's take a look at this kid average section as an example.
This is a right upper arm and a muscle that we're all pretty familiar with called the
biceps brachii.
Now, the muscles of the body are made up of hundreds to thousands of muscle cells and
if you look closely, you can probably make out string like or fiber-like cells making
up that whole muscle.
Now, muscle cells, muscle fibers can be used interchangeably so that the same thing and
forgive me, I may do that throughout the video but if I say muscle cells, same thing as muscle
fibers but these fibers have this amazing ability to contract and you can see the tendons
attaching the muscle belly or those muscle fibers up to the skeleton and even down here
and so, when they contract, generally one of the attachments is going to be more mobile
and one's going to be more of an anchor like in the case of the biceps that anchor point
is more up here and when those fibers contract, poof, we move the skeleton.
So now think of the last time that you exercise and if it's been a small season since you
exercise last, imagine your self-exercising.
So, what did you or your imaginary self do?
Maybe some of you participated in a long endurance run or went to a yoga class or played a sport
or maybe did some heavy weight training.
Now, the reason why this is important and why I'm having us think about this is is that
depending on the activity that you participated in, you are likely to engage a different percentage
of certain types of muscle fibers as compared to participating in other activities.
Think about how cool this is.
Yes, we know that a whole muscle is made up of hundreds to thousands muscle fibers but
of those total muscle fibers, some of them are going to be classified as slow twitch
and another percentage of them are going to be classified as fast twitch muscle fibers.
Yes, I will acknowledge to all of us anatomy and physiology geeks that there is an intermediate
type but we're going to mostly focus on those fast and slow twitch and this is really important
also because the slow twitch will adapt differently than the fast twitch will adapt and you have
some level of control over this depending on the act or the sports or the types of exercise
that you choose to participate in.
Now, exercising and stimulating the muscle fibers is one thing but the other side of
this is making sure that your muscles and body have all the nutrients, vitamins, and
minerals needed to have the energy to perform as well as recover after engaging all these
wonderful muscle fibers and that's why I'm excited to take a minute to discuss AG1 by
Athletic Greens.
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ounces of water and something that's important to me is that I want my workouts to count.
I want to have the energy to perform at a high level during those workouts or during
competition as well as recover as quickly as possible so that I can be ready for that
next workout and because AG1 is with those super foods, vitamins, and minerals, it can
aid in muscle recovery as well as help you feel that boost in energy.
AG1 is also NSF certified so you can be assured that what's on the label is actually found
in the powder.
If you're interested, go to athleticgreens.com/humananatomy to get started on your order and Athletic
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So, back to muscle fibers and let's start with slow twitch muscle fibers.
Slow twitch muscle fibers are actually known as slow oxidative or type 1 fibers and that
name "Slow Oxidative" implies the amazing aerobic capacity of these muscle fibers and
that's because they utilize oxygen to break down carbohydrates, fats, and sometimes proteins
in order to synthesize the energy currency of our cells called ATP or Adenosine Triphosphate.
Now, our muscle cells burn through a ton of ATP especially when we're exercising.
So, it seems like it would be a good idea to create a situation where a muscle fiber
could effectively make high quantities or high amounts of ATP and these slow twitch
fibers are set up to do this.
Slow Twitch fibers have a high number of capillaries which means they have a great blood supply
in order to receive a lot of oxygen and nutrients.
They also have a high number of these iron containing proteins called Myoglobin.
Myoglobin helps to bind and store oxygen within the muscle fibers so that that can be utilized
to synthesize ATP.
And for those of you who loved learning about the powerhouse of the cell, the mitochondria,
these slow twitch fibers are packed with a high number of these mitochondria and if you
haven't learned about mitochondria before, that's okay.
Think of these as little organelles or tiny little organs inside the cell that help synthesize
ATP and they use oxygen in order to do that.
So, as you can see, because of their great blood supply, their myoglobin content and
the number of mitochondria that they have, these slow twitch fibers are really set up
to utilize oxygen well in order to make ATP.
Now, why is oxygen so important?
Well, let me give you an idea here.
When our cells utilize oxygen to make ATP, they can make about 18 times more ATP per
molecule of glucose as compared to in a situation where the cells are making ATP without oxygen.
We generally say that per glucose molecule, we can make 36 ATP, about that, with oxygen
but only two ATP in situations without oxygen.
Now 36 ATP compared to two sounds great but there's always pros and cons or trade-offs
in the land of biology.
For example, slow twitch fibers tend to have a smaller diameter and so therefore contract
with less force and they are called slow twitch fibers because their speed of contraction
is relatively slow when compared to the fast twitch fibers and part of that has to do with
the time it takes to make that much ATP as compared to how fast the fast twitch fiber
make their ATP.
But during lower intensity activity, when there's enough time to synthesize these high
amounts of ATP aerobically, these muscle fibers are extremely resistant to fatigue.
They can take you 26 miles during a marathon.
They can help you sit in a chair, pose for extended period of time during yoga.
They can help with low weight, high repetition weight training and they can help keep you
upright all day long as the majority of your postural muscles are packed with high percent
- high percentages of slow twitch fibers.
So, think of these muscle fibers at excelling in muscular endurance and they definitely
will adapt to consistent exercise and training at certain intensities and how they adapt,
you can kind of think about that list of characteristics of these muscle fibers changing.
So, in other words, will increase the number of capillaries to these muscle fibers so we
can deliver more oxygen and nutrients.
The myoglobin content will go up.
We can bind more oxygen within that muscle fiber and the michondria will even increase
in number as well as size so we can process that oxygen and make more ATP.
One other thing that's really, really cool is that the transporters on the outside of
the muscle fiber membrane or even on the mitochondrial membrane will increase in number.
So, you can exchange things through the inside and outside of the cell as well as in and
out of the mitochondria.
So, you've essentially created this ability to get nutrients and utilize oxygen to make
even more ATP slow twitch muscle fibers and that means more muscular endurance.
But what about those fast twitch muscle fibers?
Are they great at endurance or they great at something else?
Fast twitch muscle fibers are known as fast glycolytic or type IIB fibers and for us to
understand these fast twitch fibers, we could almost reverse the facts that we learned about
the slow twitch muscle fibers because the fast twitch fibers have fewer capillaries,
lower amounts of myoglobin and fewer mitochondria.
So, these fast twitch fibers are not be known for their endurance or their ability to resist
fatigue but they are larger in diameter and to store more glycogen which is the storage
form of glucose and contract with more force and velocity.
Hence the name fast twitch or fast glycolytic.
And let me address that name Fast Glycolytic.
They're referred to as such because they undergo or typically utilize this process called glycolysis
to synthesize their ATP and glycolysis is the breakdown of glucose and in the case of
doing it anaerobically or without oxygen, we get that two ATP that we mentioned and
a byproduct lactic acid and that only takes about 18 steps.
So the benefit of that is we can get those ATP quickly but eventually we're going to
run out and you know that.
When you go for a full-fledged sprint and engage those fast twitch fibers you know that
you can only go so far before you run out of gas or run out of ATPs so to speak.
But I have to be clear; your slow twitch muscle fibers can also utilize glycolysis.
They can also utilize fats to synthesize ATP and that's a wholeer video which is going
to be awesome, we'll do that soon but if they utilize glycolysis, the big difference is
there's oxygen and what will happen is that those end products of glycolysis will get
funneled into different pathways in the presence of oxygen, something called the Kreb cycle
which you may have heard about and then going through this process of oxidative phosphorelation
and that process yields 36 ATP but it takes about 124 steps.
So, you can see how much time that takes and that's why it can only be done in lower intensities
of activity like jogging but - jogging or running compared to sprinting.
In those jogs, you can go for mile after mile after mile in the presence of oxygen.
So, again, we're kind of drilling this point home of the pros and cons of oxygen versus
not having oxygen and fast twitch versus slow twitch but let's finally get to this idea.
How do these fast twitch muscle fibers adapt?
So, in order for your fast twitch muscle fibers to even adapt, you first have to tap into
them or engage them and so, let's use this example; moving this adorable three-pound
weight or curling it and engaging my biceps.
Let me ask you a question.
To move this three-pound weight, am I engaging all of the muscle fibers of my biceps?
And the answer is no, because if every time when I engage my biceps to curl this weight,
if I engage every single muscle fiber, it would look more like this.
Coming up and I'd have to dodge it because I'd almost hit myself in the face with the
weight.
And so we need to be able to modulate the amount of muscle fibers we recruit in order
to lift weights appropriately if you will and what's really, really cool is again, if
we go back to this whole muscle that we know is the biceps as our example, yes, it's made
up of multiple muscle fibers but those muscle fibers are organized into groups or units
So, just for example sake maybe there's a unit of muscle fibers here and another unit
there and another unit there but they're kind of organized and bundled into units or groups
called Motor Units.
Now the textbook definition of a motor unit is the motor neuron on or the nerve cell and
the muscle fibers that it controls.
Now, something also that's important to note with motor units is they will only contain
a certain fiber type.
So, one motor unit would be slow twitch and another motor unit would only include fast
twitch fibers.
You're never going to have motor units that include both fast twitch and slow twitch fibers.
So, back to this example with the weight.
When I'm lifting this weight or only when a weak contraction is necessary to lift a
lightweight, your body going to recruit motor units that only include the slow twitch fibers
first and so, as I increase the weight, say I go from 3 to 10 pounds, I'll need to recruit
more muscle fibers and so therefore motor units and I may engage a couple more slow
twitch containing units but eventually when the weight or the intensity increases to a
certain point, I'll start recruiting motor units that include the fast twitch fibers.
So you can see you have to get to some level of intense to engage those fast twitch fibers
and therefore even get an adaptation and the two ways that we really increase intensity
most of the time is one, we just increase the weight or two, we increase the speed say
from going from like a jog to a sprint or maybe something like an explosive jump.
And finally, now that we've tapped into these fibers how will they specifically adapt physiologically?
Well, they tend to adapt by getting larger or undergoing a process called hypertrophy
and that is the muscle fibers that you have will increase in size and in order for them
to increase in size, you need protein to make more protein contractile units and that's
why you hear a lot about people getting protein supplements when trying to build muscle or
at least having enough protein in your diet and a muscle with a larger cross-sectional
area will produce more force.
Now, they also tend to be able to store more glycogen as an adaptation and so therefore,
you have more glucose stores in there to create more ATP even though you only create few amounts
or those two that we talked about, but you'll at least have more glucose to funnel that
into glycolysis.
And bringing this all together, I think it's important to discuss that these different
fiber types do not exist in a vacuum and neither do you.
The majority of athletic activities or different exercise routines that we participate in tend
to engage both fiber types.
Granted, you're going to engage one more than the other based on your activity of choice
and we've brought up endurance runners multiple times throughout the video.
So, yes, an endurance athlete or like a marathon runner is primarily going to engage their
slow twitch fibers but there will be times where they have to increase their speed.
Maybe they have to pass somebody during a race or they really push it towards the end
of the race and start recruiting some of those fast twitch fibers.
A sprinter, turning that sprint, fast twitch dominant but after maybe doing a recovery
run around the track or a light jog around the track, going to go more into those slow
twitch fibers and what's also interesting to note is that we have percentages of fast
versus slow in any given muscle.
I gave the example that yes, the postural muscles in general and all of us have a higher
percentage of slow versus fast so they can hold us up and endure throughout the day but
if you were to take two people and analyze say like their gastrocnemius or their calf
muscle - I was trying to mix calf and gastroc there, but if you were to analyze that muscle
one person might have a higher percentage of fast versus slow as compared to the other
person.
But what I still think is so amazing about this, depending on the activity that you like
or tend to participate in whether it's fast twitch dominant or slow twitch dominant, that
will these amazing adaptations that your skeletal muscles can undergo based on this wonderful
thing that we call exercise and luckily we have Geoffrey the skeleton here to continually
remind us of our manners and to remind us to say thank you to you all for watching all
of our crazy anatomy videos.
It really does help support the channels and so we really are grateful for that support.
Geoffrey also does want to say that you don't get a stomach forever as he is portraying
here and that you may want to take advantage of that by maybe supplementation like AG1
by Greens.
If you're interested in that, go ahead and check out that link in the description below.
If you feel the need, like, subscribe, leave some comments with the answers to those quiz
questions from earlier and also let us know about maybe any other future fitness, athletics,
or health and wellness videos that you guys might want to see in the future and we'll
see you next time.